The present invention relates generally to transmission line circuits, such as stripline and microstrip filters, and particularly to filters with resonators producing reduced cross-coupling between the resonators and thereby improving filter performance.
Bandpass and band-reject filters have wide applications in the today's communication systems. The escalating demand for communication channels dictates better use of frequency bandwidth. This demand results in increasingly more stringent requirements for RF filters used in the communication systems. Some applications require very narrow-band filters (as narrow as 0.05% bandwidth) with high signal throughput within the bandwidth. The filter response curve must have sharp skirts so that a maximum amount of the available bandwidth may be utilized. Further, there is an increasing demand for small base stations in urban areas where channel density is high. In such applications, small filter sizes are desirable.
Desirable filter characteristics are often difficult to realize for a variety of reasons. For example, energy losses due to resistive dissipation and radiation contribute to decrease in the quality factor, Q, of a filter; uncontrolled cross-coupling through radiation among the resonators in a filter tends to degrade out-of-band performance or symmetry of the frequency response of a filter.
The present invention is directed to improving the performance of the above-described filters.